1. Field of the Invention
The present invention relates to a thin film transistor and a method of fabricating the same, and more particularly, to an oxide thin film transistor in which a semiconductor thin film formed by combining an oxide semiconductor with a specific nitride is applied to a channel layer of a thin film transistor and a method of a fabricating the same.
2. Discussion of Related Art
In fabricating a thin film transistor, materials used to form a channel layer largely include silicon-based materials, ZnO-based materials, and non-ZnO-based materials. A detailed description of these materials will be provided below.
(1) Silicon-Based Material
Devices based on amorphous silicon and polycrystalline silicon are being realized. While mobility of an amorphous silicon transistor is as low as 1 cm/Vs, and that of a polycrystalline silicon transistor is as high as 100 cm/Vs, problems of device uniformity are on the rise.
(2) ZnO-Based Material
In order to enhance performance when a channel layer is formed using a ZnO-based material, methods of adjusting a ratio of zinc to oxygen (Zn/O) have been developed. Such methods include, for example, a structural approach in which a band-gap is adjusted by substituting La, Ba, Sr, etc., and adjustment through post-processes including low-temperature annealing and laser annealing.
Also, research aimed at improving deteriorated characteristics resulting from a polycrystalline structure of ZnO in a ZnO-based material is underway. For example, the characteristics of single crystallization are improved through a grain growth method and a low-temperature molecular beam epitaxy (MBE) process, and the characteristics of amorphization are improved using quaternary materials including indium, gallium, zinc, and oxygen. However, the quaternary materials including indium, gallium, zinc, and oxygen have been patented by Hosono, Japan, and thus use of them may require a license.
There has been extensive research into the improvement of characteristics by adjusting the concentration of N-type and P-type carriers when a channel layer is formed using a ZnO-based material. For example, a single-element doping method or a droplet (or drop) implantation method may be employed, a ternary compound semiconductor, e.g., ZnSnO, MgZnO or CdZnO may be used, and there is plenty of room for more research regarding additional substitutions in ternary compound semiconductors.
(3) Non-ZnO-Based Material
Non-ZnO-based materials include In—Ga—Zn—O type amorphous semiconductor and individual oxide semiconductors such as In2O3 and SnO2. The In—Ga—Zn—O type amorphous semiconductor is unlikely to be free from the original patent of Hosono. While individual oxide semiconductors such as In2O3 and SnO2 have inferior characteristics to ZnO, they have not undergone sufficient research and there is room for improvement of their characteristics by composition adjustment and doping substitution. Non-ZnO-based materials include opaque semiconductors such as CdS, ZnS, ZnSe, etc., whose characteristics are applicable to active matrix organic light emitting diodes (AM OLED), but whose opacity is technically restrictive.
As described above, thin film transistors using silicon-based materials, ZnO-based materials, and non-ZnO-based materials exhibit the following problems.
A ZnO-based thin film transistor's characteristics are susceptible to changes in humidity, annealing processes, and manufacturing processes, and thus it has problems of reliability. Further, the transistor has a crystalline channel which may raise problems of device uniformity. Moreover, device deformation caused by current and light may be significant.
Further, in the case of a thin film transistor employing an IGZO channel in which indium and gallium oxides are applied to ZnO, indium and gallium are relatively scarce and therefore costly materials.
In a silicon-based thin film transistor, in particular, amorphous silicon, mobility is low, and with polycrystalline silicon, uniformity may be a problem in creation of a large panel. In particular, the amorphous silicon transistor is vulnerable to instability depending on current.
In general, an oxide thin film transistor may be vulnerable to instability depending on current due to the inside of a channel thin film or an interface with a gate insulating layer.
In addition, a thin film transistor using ZnS, ZnSe, CdS, etc. is opaque and thus may not be applicable to transparent electronic devices.
In the course of research into thin film transistors, the present inventors found that when an oxide semiconductor is combined with a specific nitride to form a channel layer, mobility is increased and stability is ensured at a high temperature. These findings led to the completion of the present invention.